The invention relates generally to wireless communication systems, and more particularly to an apparatus and method for estimating a channel phase in a digital radio frequency communication system.
In a typical wireless communication system such as a digital radiotelephone system, a base station having a controller and a plurality of transmitters and receivers communicates with mobile stations operating within an area served by the base station. Multiple access wireless communication between the base station and the mobile stations occurs via radio frequency (RF) channels which provide physical paths over which digital communication signals, such as voice, data and video are transmitted. Base-to-mobile station communications are said to occur on a forward-link channel, while mobile-to-base station communications are referred to as being on a reverse-link channel. Well known RF digital channelization techniques include, for example, time division multiple access (TDMA) and code division multiple access (CDMA) and other channelization techniques. Communication systems using frame based CDMA channelization are described, for example, in TIA/EIA Standard IS-95.
Transmitting a communication signal over an RF channel through a medium such as air causes a received communication signal to significantly differ from an originally transmitted communication signal. A transmitting communication signal may be altered during transmission over a channel resulting in channel gain and channel phase shifts. In addition, additional noise may be introduced.
A quality of recovery of transmitted communication signals from the received signal may be enhanced by accurately estimating channel parameters such as channel phase and other parameters. Pilot signals have been implemented to allow a determination of channel phase. Symbols communicated over a pilot channel typically communicated with symbols of the signal of interest and received over the same antenna under a different code (in a CDMA system, for example).
In spread spectrum systems, for example, such as CDMA systems, there is a need for channel phase compensation due to channel phase shifts due to propagation of information through the channel. A polarity of symbols can be affected by channel phase. As such, coherent receivers, those which receive information as conveyed by polarity of the transmitted symbols, may incorporate some type of channel phase compensation.
One known radiotelephone receiver having channel phase compensation, is disclosed in U.S. Pat. No. 5,790,596, issued on Aug. 4, 1998, entitled xe2x80x9cRadiotelephone Communication Unit Displaying Chronological Information,xe2x80x9d having inventor Thomas A, Sexton, and assigned to instant assignee. Although such a receiver has many advantages, such receivers generally attempt channel phase estimation for slowly changing channels, but not typically for channel parameters that can change throughout a frame. In addition, typical phase compensation techniques are used which provide for reencoding of future symbols based on some type of channel phase estimate of previous symbols. However, such systems may not adequately provide channel phase estimations and allow reprocessing of the same symbols within a frame based on the estimated channel phase shifts.
Consequently, there exists a need for a receiver and method that facilitates improved channel phase shift estimation and can allow reprocessing of symbols in the same frame that have previously been corrected for channel phase shifts.